An experiment that last year claimed to have observed faster-than-light particles may have been undermined by a faulty fibre optic cable.
Last September, LHC operator CERN announced that its OPERA experiment had discovered neutrinos travelling 0.002 percent faster than the speed of light.
Einstein had said nothing could ever travel faster than light, and doing so would be like traveling back in time.
But James Gillies, a spokesman for CERN, said on Wednesday the lab's startling result was now in doubt.
Earlier on Wednesday, ScienceInsider, a website run by the respected American Association for the Advancement of Science, reported that the surprising result was down to a loose fibre optic cable linking a Global Positioning System satellite receiver to a computer.
Gillies confirmed that a flaw in the GPS system was now suspected as a possible cause for the surprising reading. Further testing was needed before any definite conclusions could be reached, he added.
The faster-than-light finding was recorded when 15,000 neutrino beams were pumped over three years from CERN to an underground Italian laboratory at Gran Sasso near Rome.
"A possible explanation has been found. But we won't know until we have tested it out with a new beam to Gran Sasso," Gillies told Reuters in Geneva.
Physicists on the experiment, called OPERA, said when they reported it last September that they had checked and rechecked over many months anything that could have produced a misreading before announcing what they had found.
A second test whose results were announced in November appeared to provide further evidence that neutrinos were travelling faster than light. But many experts remained sceptical of a result that would have overturned one of the fundamental principles of modern physics.
Gillies said CERN would be issuing a full statement early on Thursday.
(Reporting by Bob Evans in Geneva and Kate Kelland in London; Editing by Peter Graff)